Help with 500c Phase Inverter Noose Removal

Buff - I noticed you have the Fisher set for 4 ohm speakers. Is that correct? Your speakers are 4 ohms? Thorne
 
Hi Thorne,

Yes, I've been testing with Infinity 5000 which are 4 ohm. I've also reconfigured and tested with Dynaco A25 which is 8 ohm. I prefer the sound of the Inifinitys, but the WAF is much greater for the Dynacos, and this set is destined to leave the man cave and take up residence in the library.
 
And that's the answer. In the first pic, you were using 90% modulation, while the the last two were taken with 30% modulation. But while that explains the difference in your pics, it also highlights that it takes a 90% modulated signal from your tuner to produce an output waveform that is equivalent to mine that is only 30% modulated.That would tend to indicate potential issues with Z5 either in alignment or serviceability. If it truly is an alignment issue, then the entire IF strip should be aligned in resolving the issue.

Dave
 
Dave, glad the clarification helped make sense of the pictures. I had earlier reported that V1 had tested poorly on my tube tester. By inserting a signal at TP1 that still resulted in a distorted waveform, does that take V1 off the hook? At least for this problem?

Once the power switch issue is resolved, I would like to go back and input a MPX signal. I got a clean stereo signal out of the tuner previously, but I paid so little attention to the tuner section while working on the power amp that I really don't know how well it was performing before. That would at least help confirm whether the tuner problem began during/since the restoration.

I disassembled the power switch and now understand why the arcing. One of the switch contacts on the rotor is loose and doesn't remain firmly in contact. It will require a new switch. I've inquired with Mark Oppat about a replacement, but the one he has on the auction site is not the right type. A possibility would be to install a relay and use the remaining switch contact (currently wired to the aux plugs) to operate it.

BuffDriver
 
He may still have it correct switch. Take a picture of yours and send it to him. Fisher used at least two different types of switches (one snaps on the back of the volume control, the other mounts with three tabs that are bent over inside the cover of the rear control, requiring disassembly of the control), so the pic will help him to know for sure the type you need, and whether he has it or not.

The recovered audio is captured solely in Z5, so if the audio is weak for a given modulation level, then that is the point of concern -- particularly if the signal strength meter is indicating a strong signal at the grid of the last limiter stage. Before Z5, everything operates at either an RF or IF frequency.

Dave
 
Got it! I'm investigating solutions to the switch now. I would like to keep things as stock as possible. I will report back on the tuner issue once the switch is resolved. Meanwhile, I guess that I have time to work on the wood case now...
 
So it turns out that tuners don't work well with one of the B+ wires broken. :banana:

I felt sure that the tuner was working better before than after my work on the phase inverter. I began checking connections and evaluating what I could have goofed up. Somewhere during the phase inverter noose removal, I stressed the wire leading from the PI adjustment pots to the FM circuitry and it broke inside the insulation. Once I replaced that wire, FM tuning returned to normal.

I still have to work on the power switch, but it sure feels nice to know that the tuner is working fine.

Thanks to Dave and all who have offered help. I will be back to post a wrapup once the switch is repaired.

BuffDriver
 
I'm kind of amazed the tuner was working at all with no B+. Must have been arcing through and getting a wee bit of voltage.
 
It was missing the 350V tap that connects at the junction of R71, R75, and R77. Just darned glad that everything's okay now. Here's what it looks like now. 0.25% THD in mono.

SDS00006.jpg SDS00007.jpg SDS00008.jpg IMG_5876.jpg
 
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I should add that despite my buffoonery, I learned a lot about the 500C and about troubleshooting tuners. Again, I'm so indebted to Dave Gillespie.

Also, @notdigital has been helping me work through the switch issue. With his help and this thread, I disassembled the switch, removed the offending contacts, cleaned the remaining contacts, and re-assembled the switch. I've wired the plugs in back to be non-switched and have wired the receiver power into the good contacts. There is a CL-80 installed and I plan to use a power strip to power up/down the receiver. If I should run across a good deal on a switch, I may consider putting in a new one. Here are some internal pics of the switch.

IMG_5872 (1).jpg IMG_5871.jpg

This is all that's left of the failed contact:
IMG_5874.jpg
 
Ah ha! So what was really going on was that the diode matrix circuit was not receiving any bias voltage so as to bias the appropriate set of diodes "on", allowing them to pass un-distorted audio. Hence, the severe cross-over distortion noted in one of your earlier scope shots.

You should note however that there is an error on the schematic, in that the 350 volt source actually only connects to R71 and R75 in this section of the receiver. R77 -- shown also connecting to the 350 volt point on the schematic -- actually connects to the relay contact where R64 and R71 connect to, which then allows the circuit to operate properly based on the state of the relay contacts.

Congrats on finding the problem and getting it going again!

Dave
 
Dave G will explain it in glorious detail. He did explain it in this thread: http://audiokarma.org/forums/index.php?threads/fisher-400-service-bulletin.671281/
I'm a novice electronics guy so I'll explain it in extremely "lay" terms. The noose limits power at high output volumes. As I understand it it was designed into the receiver to prevent distorted audio at those high volumes. Simply removing the noose is not enough because, as you would guess, doing so affects the rest of the circuit.

I struggled with this mod because also required after removal of the noose is readjusting the phase inverters (PI's) (http://www.audiokarma.org/forums/index.php?threads/quick-and-easy-400-phase-inverter-adjust.559415/). My problem was that with the existing resistors attached to pins 1 and 3 on tubes V12 and V13 (in a 500-C), I couldn't adjust the PI's so that the resistances at pin 1 of each tube equaled the resistance at pin 3. I had to replace the 68K pin 3 grounded resistors with 45K resistors so that adjusting the PI's made it possible to set both resistances equal. And in doing all of that I broke the pot for one of the PI's !

But it was all worth it because the sound was greatly improved at louder volumes. Whether or not my observed improvement is real or imaginary because after all that I WANTED the result to be an improvement...well, I doubt anyone would waste their time debating me on the subject...LOL !!

Thorne
 
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I did read Daves great observation , but before accepting suggested improvements over what has been the product of clever pioneering engeneers focused on the entire concept, I ask questions, such as:
1)) this thread showed that releasing "the noose" leads to increase THD.
2)is the need to reduce plate V on amp stage a compensatory noose to counter the first?
3) is the "sounds better" a result of these harmonics?
4) I am ready to be convinced, but I need hard evidence ( measurements). Just more output power is not e nough for me, since the 500c. I work on has plenty. I would enjoy the beating.
 
1. Removing the noose does not lead to increased THD. In the case it was mentioned that it did in this thread, the full modification had not been performed at that point. Once the modification was completed, distortion was no higher than before, and is usually less.

2. Lowering the voltage on the AF amplifier stage allows the inverter stage to bias itself properly within the linear portion of its operating curves.

3. No. See below.

The noose performs two functions: (1) It limits AC drive to the output stage, possibly as a protection element for the output tubes, and (2) It allows the inverter stage to bias itself below saturation levels in spite of the higher than normal plate voltage offered by the previous stage that is direct coupled into it. As such it was a move to maximize gain production in the AF Amplifier stage, although at the expense of greater than normal losses through the inverter stage due to the noose causing it to bias itself so cold.

The problem with the noose is therefore two fold: (1) For output tubes requiring an unusually high amount of grid bias voltage, it puts the maximum power drive requirements of the output stage outside of that which the stock inverter design is capable of, causing increased distortion at higher power output levels, and (2) Because the inverter section biases itself so cold, many Russian based 12AX7 tubes will not operate properly in the stock design since the quiescent operating point is so low on the load line -- a point at which characteristic consistency from one tube to another varies considerably. Russian 12AX7s are very good tubes when operated throughout the linear portion of the operating curve. But near the cut-off end of the curves, the Russian tube characteristics vary enough -- as compared to the original American pieces -- as to be problematic in the stock design.

By removing the noose then: (1): The inverter stage is able to produce plenty of low distortion drive to accommodate output tubes requiring even very high bias voltage levels, and (2), The stage will operate properly with American or Russian based tubes, because the inverter stage then operates well within the linear portion of its grid curves. The OLG (Open Loop Gain) of the overall design remains unchanged, as the loss of gain in the AF Amplifier stage due to operating it from a slightly lower B+ source is more than made up for by eliminating the excessive losses in the original inverter stage since it is now operating well within the linear portion of its curves. The result is that distortion -- when properly implemented -- is invariably reduced.

The modification then has nothing to do with trying to outwit the clever pioneering engineers of Fisher, but rather, adapt the circuit to be more in align with the reality of today's vacuum tube audio environment.

Dave
 
I've had problems with Russian tubes at very low current levels myself. The amp I had issues with was a completely different design, but it ran at similarly low current levels. At higher volume the tube would actually cut off completely. Sound output would drop for a second and then come back. If the volume wasn't reduced it would basically "pulse" the output in and out every half second or so. Any old production tube worked fine in that circuit, and those Russian tubes worked fine anywhere else.
 
Great beating! thank you. How can I observe or document the change when I do it in the 500c? Would be nice to mod one channel and compare to the other. what level of clipping is too early?
Is comparing THD in both channels an indicator.? Is picturing the spectrum of a 1k signal a usefull comparison? My ears are not good enough.
 
While somewhat off-topic, one of my projects for this summer is to learn FM radio, how to fix them and how to align them as I am getting so many receivers of late. Is there a recommended source or video that would help further my education? Thanks!
 
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